Use h1 headers in readme

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

README.md

-### gitlab-shell: ssh access and repository management
+# gitlab-shell: ssh access and repository management
 GitLab Shell is an application that allows you to execute git commands and provide ssh access to git repositories. It is not a unix shell nor a replacement for Bash or Zsh.